This invention relates to the field of motion control, and deals with servomechanisms which accurately control the position or velocity of a moving object. The invention can be used to control objects whose motion is either linear or rotational.
In various industrial processes, it is necessary to control the linear or angular motion of an object. For example, a motor may be used to control the position of a robot in a factory. Such control is accomplished by controlling precisely the angular position of a rotating shaft.
In another example, in tuning a magnetron, it is necessary to adjust remotely and precisely the position of a wall of a resonant cavity. A controlled linear motor can be used for this purpose.
In the prior art, it has been known to control the angular position of a rotating shaft of a motor, by providing position and velocity feedback. Velocity feedback is obtained by continuously monitoring the angular position of the shaft, and converting this information into data on its angular velocity. A signal representing the estimated angular velocity is fed back to the input side of the power amplifier which drives the motor.
The simplest and most obvious way to obtain an estimate of velocity is to differentiate the position signal. This procedure has been done, and it works reasonably well. However, electronic differentiation is an inherently noisy process, especially when the object is moving very slowly. Even where the object has been successfully brought to its new desired position, the "noise" generated by the differentiation circuit produces extraneous current in the motor windings, resulting in unwanted heating. It also may cause spurious motions in the object itself.
Another method of estimating the angular velocity is to connect a tachometer to the object. This technique is often used to estimate the angular velocity of a rotating shaft. Examples of patents using such tachometers for velocity feedback include U.S. Pat. Nos. 4,341,986 and 4,345,194. A tachometer can reduce the noise associated with an ordinary differentiator, but it does significantly affect the cost and reliability of the apparatus, and adds to the cost of installation.
An all-electronic estimator of velocity has also been considered, in the prior art. The paper by Jacob Tal, entitled "Velocity Decoding in Digital Control Systems", of the Proceedings of the 9th Symposium on Incremental Motor Control Systems, 1980, discusses the possible use of a Kalman filter as part of a motor control circuit. However, the paper does not give details of the circuitry that would be used, and also states that the use of a Kalman filter is not practical unless a detailed model of the motor is available. The cited paper also suggests that this approach would require an unduly large amount of computation time.
The present invention provides a velocity estimation circuit which replaces the tachometer used in the prior art. The invention is completely electronic, and it can be implemented in either analog or digital form. The invention can thus reduce the cost of motor control circuitry by as much as about 15-20% of the cost of the motor and control units. Moreover, the device of the present invention operates by integration, not differentiation. The present invention therefore substantially reduces the electrical noise which is an inherent characteristic of differentiators.